He is the one who united mankind! He gave us a purpose when we had none and he shielded us from the horrors you worship! He formed an empire that is eternal and he still watches over it! I am the one who defends it from you! Your gods are the ones who are fools, thinking that they can enslave mankind!
Your Imperium is built on a foundation of lies and blasphemies! It is a rotting edifice of sin and heresy that must be cleansed! Mankind is enslaved by the false light of the Astronomicon, and Chaos comes to free us, and make us far more than men!
What you call "hummanity" is blasphemous and weak! Mankind must leave behind the chains of it's current form. Death is but the beginning. Mankind is destined to embrace the Warp as the next phase of evolution. Those who cling to the lies and blasphemies of the False Emperor must be punished for their sins.
What's the difference between surrendering your humanity to the gods than it is to surrender it to a mutant? The Emperor is a psyker, a psyker is a mutant, just because he thinks he has your best interests at heart does not change that. All you're doing is exchanging one dictatorship for another.